This invention relates to a process for coating one or both surfaces of a polyimide sheet with electrolessly deposited nickel or cobalt and electrolytically deposited copper, and to the thermally stable laminate product obtained by the process.
Certain electronic assemblies have conductor traces on both sides of a dielectric substrate. Preferred dielectrics for applications requiring flexible substrates, especially in multilayer constructions, involve the use of polyimide films. In general practice the conductive layers on this substrate are provided through the use of metal foils and adhesives specially formulated for their physical and thermal stability. The conductive layers are also provided in some cases through direct metalization by sputtering or by electroless deposition involving methods well known to those versed in the art.
Currently there are no commercially available polyimide laminate materials produced in the absence of an adhesive, suitable for IPC Class 3 electronic circuit applications. The adhesive bonding method has certain disadvantages especially in critical service and multilayer applications, where either properties of the adhesive or the physical space occupied by the adhesive are limiting factors. For example, these adhesive-bonded laminate materials exhibit poor dimensional stability, a severe disadvantage for laying up multilayer boards. The preferred construction (especially in multilayer applications) would avoid the use of adhesives and provide the metal directly bonded to the substrate. Until the present invention, however, the methods available for direct metalization of polyimide film substrates had more significant drawbacks than the adhesive bonding methods.
Two methods for preparing polyimide adhesiveless metal coated laminates are the related methods of vacuum and sputter deposition, followed by electrolytic copper buildup to desired thicknesses. Sputtering provides better adhesion than vapor deposition but neither technique provides physical properties adequate for critical service applications. Several commercially available single-sided materials are produced by these methods. The adhesion values obtained on subsequent copper buildup to greater than 25 .mu.m are very low for materials with sputtered copper. One structure has an initial chromium layer sputtered to the polyimide film which is then covered with a sputtered layer of copper is described in U.S. patent application Ser. No. 775,918. A disadvantage of this construction is that chromium is not removed easily during subtractive processing of electronic traces using standard etchants designed to remove copper.
An alternative method for preparing a polyimide adhesiveless metal clad laminate is to cast a liquid solution of a polyamic acid onto the surface of a metal foil, and to heat the entire composition to a temperature which will imidize the polyamic acid and form a polyimide or amide modified polyimide film. Several modifications of this basic technique are disclosed in U.S. Pat. Nos. 3,682,960; 3,781,596; 3,981,691; 4,148,969; 4,496,794 and 4,675,246. At present, laminates produced by this technique have limited application areas due to their poor dimensional stability. This method is also limited by the availability of suitable foil thicknesses.
In order to provide an adherent metal layer or layers to a polyimide film substrate by chemical metal deposition, several basic procedures are employed. Since processing is done in aqueous solutions, the surface of the polyimide film must be rendered hydrophilic to facilitate uniform adsorption of the catalyst used for seeding the electroless metal deposition. Commonly referred to as etching, this treatment also serves to microetch the surface of the polyimide, thus providing a mechanical interlock between the polyimide and metal layer. This surface preparative step will be referred to herein as texturing for purposes of this application to avoid confusion with the removal of unwanted metal during circuit preparation which also is referred to as etching in the printed circuit industry.
U.S. Pat. Nos. 3,791,848 and 3,821,016 to DeAngelo disclose an aqueous ethylenediamine composition wherein the diamine is present in an amount in excess of its degree of solubility, thereby forming a two-phase system. The preferred aqueous basic compounds are disclosed as the hydroxides of the alkali metals including sodium, potassium, lithium, rubidium, and cesium. The examples are specifically directed to texturing with sodium hydroxide in very strong solution containing the ethylenediamine in suspension so as to yield a pitted surface on the polyimide. Since a two-phase system is utilized, the polyimide surface tends to be non-uniformly and incompletely textured. The degree of texturing for a given portion of the surface depends upon which phase of the solution to which a portion of the surface is primarily exposed. These non-uniformities result in subsequent non-uniform deposition of catalyst and electroless metal and in a rough irregular appearance of the final metal coated film, and variable bond strengths of subsequently depsited metal layers.
U.S. Pat. No. 4,517,254 to Grapentin et al claims the use of various metal chelators primarily EDTA (ethylenediamine tetratetraacetic acid) derivatives, and NTA (nitrilo triacetic acid) in dilute caustic solutions as treating agents to prepare polyimide thin films for metalization. Grapentin suggests a long list of many possible amine compounds which could be combined in mixtures containing caustic, but no specific examples are provided other than the above, to determine which mixtures would provide benefit, and to what extent. It is known from DeAngelo that one could combine amines with caustic to affect the polyamide surface, but Grapentin provides no peel strength data, or description of the surface topography of his treated films. Grapentin does not describe the problem or propose a solution to treatment of film by two phase mixtures. The Grapentin patent therefore does not address the subject of a uniformly and completely textured surface and does not clearly identify that his process does indeed provide a useable laminate material for class three circuit application.
U.S. Pat. No. 3,767,538 to Politycki et al describes an attempt to produce a well-adherent double-sided laminate on polyimide film. The film surface is roughened by a fresh mixture of sulfuric and hydrochloric acids or by mechanical impingement of sand on the surface and, if needed, a further sodium hydroxide treatment may be employed. The polyimide film is then heated to expel water and then seeded for metalization in a colloidal palladium bath. After this, an electrically conductive, water-vapor permeable continuous layer of silver is deposited by electroless deposition and the film is heate at 150.degree. C. to expel water. Finally, a layer of copper is electrolytically deposited to form the laminate structure. No specific adhesion values are reported for the laminate produced by this method.
A high speed additive circuit process on a polyimide substrate using electroless nickel for metalization (no etching of metal required) is described in U.S. Pat. No. 3,573,973 to Drotar et al. The basic process steps are: preparing the substrate, catalyzing, printing a mask, heat setting of the masking ink, electroless plating of a nickel/phosphorous alloy, heating at 195.degree. C. to improve metal to polyimide bond and then either electrolytic plating or solder coating to decrease the electrical resistance of the resultant structure. The use of a nickel/phosphorous alloy as the metalizing layer may cause difficulty etching laminates produced by this method during subtractive circuit production methods. The adhesion values is no mention of the stability of the product to solder float or thermal cycling stress. The low adhesion of metal to polyimide limits the use of these laminates to specialty applications. The method of Drotar, therefore, does not provide a method for preparation of metal coated laminates for general use in the printed circuit industry.
In U.S. Pat. No. 3,954,570 to Shirk et al, another additive method is disclosed wherein the electroless deposit may be nickel or another metal. The authors cite no advantage of one metal over another in the initial metalizing step, and specifically claims a patterned flame sensitized surface which is catalyzed, preferably with tin and palladium, then electroless plated with either copper, nickel, or cobalt. The method of Shirk also provides low bond strengths, and for the nonstandard solder dip test disclosed, no advantage is recognized for the use of electroless nickel rather than electroless copper or cobalt.
Another additive method for circuit preparation on a polyimide substrate is disclosed in U.S. Pat. No. 4,078,096 to Redmond et al. The method described is for texturing the surface of the polymer with a hydrazine/caustic solution, catalyzing, then plating with either electroless nickel, copper or cobalt. The method relates to the preparation of a specific circuit type by an additive process, and the maximum initial peel strength disclosed is only 4.6 lb./in. which is too low for general use in the printed circuit industry. Samples made by the method of this patent result in cohesive failure of the polyimide film at bond values of only 4.0 lb./in. using the optimum formulation of 60 percent hydrazine hydrate and 9.3 percent sodium hydroxide disclosed by Redmond in Table II. This lowering of the cohesive strength of the film is alluded to by Redmond in Column 4, lines 37-40, where the failure mode of well adhered and poorly adhered samples is discussed. Cohesive failure at such low values indicates degradation of the polymer film which is seen as ripping in the Z-axis during peel testing. There is no disclosure of completely coating one or both surfaces of a polyimide sheet with electrolessly deposited metal followed by electrolytically deposited metal.
There have been prior attempts to provide an adherent metal layer on both sides of polyimide sheets by direct metalization using electroless copper followed by electrolytic copper buildup to the desired thickness. This technique has proven unsuitable in practice since blistering of the electroless copper layer usually occurs when simultaneously applied to both sides of the film. If blistering is avoided, the peel strength of the resultant laminate is less than that for a single-sided laminate of the same metal thickness.
Perrins, in Transactions of the Institute of Metal Finishing (1972) Volume 50, pp. 38-45, discloses a process for electroplating propylene polymers with electroless nickel or copper followed by electrolytic plating with copper. The plating in this case in conducted on only one side of propylene copolymer plaques. The use of the nickel provides improved adhesion especially after thermal cycling of the sample which will cause degradation of the copper/polymer bond. This process, where nickel is used as the base metal layer, has only been applied to injection molded or bulk processed polymers, but has not been extended for use on thin film substrates. Therefore, the nickel process has been restricted to use where the nickel layer is never removed but remains a permanent part of the finished article.
In "Applications of Additive Circuitry", a technical paper presented at the Institute of Printed Circuits in September, 1974, Brewer discloses a method in which nickel is deposited on both surfaces of a paper reinforced phenolic. Electroless copper is then deposited and the plated panel is heated beyond the glass transition temperature of the substrate material to improve the nickel to polymer adhesion.
In Plating and Surface Finishing, "Interfacial Bonding of Nickel to Polyamide-Imide", Vol. 66, No. 6, (June, 1979) pp. 68-71, Levey et al, describes nickel plating on a rigid polyamide-imide substrate textured with an abrasive and/or sodium or potassium hydroxide. The article relates the effect of various surface treatments on metal adhesion, and specifically to the combination of mechanical abrasion followed by a chemical treatment such as dipping in aqueous alkali metal hydroxide solutions. The article states that no advantage was observed with the use of nickel rather than copper for the initial metalization of the polymer, and concludes that the adhesion of the metal to the polyamideimide substrate is a combination of both mechanical and chemical factors related to the substrate.
Prior to the present invention, there has been no commercially viable method available for the direct coating of polyimide films with electrically conductive layers, without the use of an adhesive exhibiting adequate properties for general use in electronic circuitry. Adhesiveless laminates with metal on both sides have not been available due to the tendency for an electroless metal layer to blister during deposition onto both sides of a thin film, and to the destruction of polyimide to metal adhesion by thermal shock upon immersion in molten solder or because of undesirable reduction of film strength due to texturing. The problem of electroless metal blistering is especially acute in the complete metalization of thin polymeric film; hydrogen evolution during the deposition process has been suggested as a cause of electroless metal blistering. Electroless metal blistering on thin polymeric substrates is greatly reduced when the metal deposition is restricted to certain areas on the film (i.e., other than complete surface coverage).
In copending application Ser. No. 018,346, filed Feb. 24, 1987, a process is disclosed for forming polyimide film coated on both surfaces with electroless nickel or cobalt and copper coated on the nickel or cobalt which is useful for forming printed circuits. The resulting laminate is capable of passing IPC tests for peel strength and solder float resistance. However, the product is inconsistent in passing modified IPC tests for thermal cycling stability wherein the product is subjected to a plurality of high temperature environments. Such a thermal cycling environment would be experienced by such a laminate product that forms a layer in a multi-layer laminate.
It would be desirable to provide a process for coating at least one surface and preferably both surfaces of a polyimide film with electrolessly deposited metal, followed by electrolytically deposited copper having sufficient adherent strength to the polyimide and sufficient resistance to thermal shock to render the resultant laminate useful for fabricating electronic circuits.